Metabolički sindrom izazvan ishranom bogatom fruktozom : uloga signalnih puteva regulisanih glukokortikoidnim hormonima u visceralnom masnom tkivu i hipotalamusu pacova

Abstract

Upotreba zašećerenih napitaka bogatih fruktozom predstavlja jedan od glavnih uzročnika povećanja prevalence gojaznosti i pridruženih metaboličkih poremećaja. Pokazano je da je fruktoza uključena u razvoj i progresiju metaboličkog sindroma putem poremećaja regulacije metaboličkih puteva u hipotalamusu i masnom tkivu, kao glavnim organima zaduženim za kontrolu unosa hrane i energetskog metabolizma. Takođe, poznato je da je ishrana bogata fruktozom povezana sa stanjem leptinske rezistencije u hipotalamusu. Postoje i dokazi da pojačana regeneracija glukokortikoida, posredovana enzimom 11β hidroksisteroid dehidrogenazom tipa 1 (11βHSD1), može doprineti razvoju adipoznosti i metaboličkih poremećaja. Glukokortikoidni hormoni su uključeni u regulaciju homeostaze triglicerida u masnom tkivu i mogu da modulišu i proces lipolize i proces lipogeneze putem povećanja ekspresije lipolitičkih enzima, kao što je lipaza osetljiva na dejstvo hormona (eng. hormone-sensitive lipase, HSL) ili posredstvom drugih regulatora metabolizma lipida, kao što su fosfoenolpiruvat karboksikinaza (eng. phosphoenolpyruvate carboxykinase, PEPCK) i lipin-1. Proces adipogeneze u masnom tkivu odvija se uz učešće brojnih transkripcionih regulatora koji stimulišu ekspresiju gena lipogeneze među kojima se nalazi i gen koji kodira γ izoformu receptora koji aktivira proliferaciju peroksizoma (eng. peroxisome proliferator-activated receptor γ, PPARγ) i protein koji se vezuje za element regulisan sterolom (eng. sterol regulatory element-binding protein-1, SREBP-1). Poremećaj funkcije masnog tkiva u metaboličkom sindromu može biti i rezultat hronične inflamacije koja se karakteriše akumulacijom makrofaga u masnom tkivu i povećanom sekrecijom proinflamatornih citokina. Pretpostavili smo da se efekti glukokortikoidnih hormona na razvoj visceralne adipoznosti nakon dugoročne ishrane obogaćene fruktozom, ostvaruju putem promena u leptinskoj osetljivosti u hipotalamusu i putem promena transkripcionih regulatora adipogeneze u visceralnom masnom tkivu mužjaka pacova soja Wistar. U tom cilju analizirani su efekti ishrane obogaćene 10% i 60% rastvorom fruktoze u trajanju od 9 nedelja na visceralnu adipoznost, dislipidemiju, insulinsku i leptinsku osetljivost, histološke promene u masnom tkivu i koncentraciju kortikosterona u plazmi i tkivu. Leptinska senzitivnost je analizirana određivanjem koncentracije leptina u plazmi, određivanjem proteinskog nivoa leptinskog receptora (Ob-Rb), kao i analizom nivoa iRNK za inhibitorni protein SOCS3 (eng. suppressor of cytokine signaling) i neuropeptid Y (NPY). Glukokortikoidna signalizacija u hipotalamusu i visceralnom masnom tkivu je analizirana na nivou prereceptorskog metabolizma, kao i na nivou ekspresije i subćelijske raspodele glukokortikoidnog receptora (GR). Takođe, izmereni su i nivoi iRNK za PEPCK i HSL kao ciljnih gena GR-a. Pored toga, određeni su i nivoi PPARγ, SREBP-1 i lipina-1 kao ključnih transkripcionih regulatora uključenih u procese adipogeneze i lipogeneze. U cilju procene inflamatornog stanja u masnom tkivu analizirani su proteinski nivo i subćelijska raspodela nuklearnog faktora kappa B (eng. nuclear factor kappa B, NFB), kao i nivoi iRNK za TNF-α, IL-6 i MIF. Rezultati ove doktorske disertacije su pokazali da primenjene dijete obogaćene fruktozom ostvaruju različit uticaj na razvoj visceralne adipoznosti, kao i na signalne puteve leptina i glukokortikoida. Povećanje količine visceralnog masnog tkiva je zapaženo samo nakon ishrane pacova 60% rastvorom fruktoze i praćeno je smanjenim nivoom leptinskog receptora u hipotalamusu i povećanim nivoima iRNK za SOCS3 i NPY. Zanimljivo je da je ishrana obogaćena 10% rastvorom fruktoze dovela do pojačanja glukokortikoidne signalizacije i lipolize u masnom tkivu, dok je ishrana obogaćena 60% rastvorom fruktoze dovela do utišavanja glukokortikoidne signalizacije i stimulacije transkripcionih regulatora adipogeneze. Pored toga, ishrana obogaćena fruktozom nije dovela do aktivacije transkripcionog regulatora NFκB i nije uticala na promenu proinflamatornih citokina u masnom tkivu, bez obzira koja koncentracija fruktoze je bila primenjena. Na osnovu dobijenih rezultata može se zaključiti da dugotrajna ishrana obogaćena fruktozom dovodi do pojave leptinske rezistencije u hipotalamusu i razvoja visceralne adipoznosti, ali samo pri visokim koncentracijama fruktoze. Rezultati su ukazali da efekti ishrane obogaćene fruktozom na pojavu adipoznosti najverovatnije uključuju međusobno zavisne efekte leptina i glukokortikoida na nivou centralnih puteva uključenih u kontrolu energetskog bilansa.

The rise in consumption of refined sugars high in fructose appears to be an important factor for the development of obesity and metabolic syndrome. Fructose has been shown to be involved in genesis and progression of the syndrome through deregulation of metabolic pathways in the hypothalamus and adipose tissue as major organs for control of food intake and energy metabolism. Fructose consumption has been previously associated with the state of leptin resistance in the hypothalamus. Furthermore, there is evidence that enhanced glucocorticoids regeneration, mediated by the enzyme 11β hydroxysteroid dehydrogenase type 1 (11βHSD1), may contribute to adiposity and metabolic disease. Glucocorticoids are involved in the regulation of triglyceride homeostasis in the adipose tissue and can modulate both lipolysis and lipogenesis through increased synthesis of lipolytic enzymes, such as hormone-sensitive lipase (HSL), or through other regulators of lipid metabolism, like phosphoenolpyruvate carboxykinase (PEPCK) and lipin-1. During expansion, adipose tissue responds by increasing adipogenesis through a complex regulatory network of transcription factors including peroxisome proliferator-activated receptor γ (PPARγ) and sterol regulatory element-binding protein-1 (SREBP-1), both involved in stimulation of lipogenic gene expression. Adipose tissue dysfunction in metabolic syndrome may result from chronic inflammation characterized by accumulation of adipose tissue macrophages and higher secretion of proinflammatory cytokines. We hypothesized that glucocorticoid signalling is involved in the effects of different long-term fructose diets on the development of visceral adiposity through alterations in hypothalamic leptin sensitivity and adipogenic transcription factors in the visceral adipose tissue of male Wistar rats. We analyzed the effects of 9-week drinking of 10% and 60% fructose solutions on visceral adiposity, dyslipidemia, insulin and leptin sensitivity, adipose tissue histology and both plasma and tissue corticosterone levels. Leptin sensitivity was analyzed by measuring plasma leptin concentrations, hypothalamic leptin receptor (Ob-Rb) protein level and suppressor of cytokine signaling (SOCS3) and neuropeptide (NPY) mRNA level. Glucocorticoid signalling was assessed in both hypothalamus and visceral adipose tissue at the level of prereceptor metabolism and at the level of glucocorticoid receptor (GR) expression and compartmental redistribution. The level of expression of GR target genes PEPCK and HSL was also determined. In addition, we analyzed the levels of PPARγ, SREBP-1 and lipin-1 as a key transcriptional factors involved in adipogenesis and lipogenesis. To examine adipose tissue inflammatory state we analyzed protein level and intracellular redistribution of nuclear factor kappa B (NFB), as well as mRNA levels of TNF-α, IL-6 and MIF. The results revealed different impact of applied fructose diets on visceral adiposity, leptin and glucocorticoid signalling. Only the rats kept on 60% fructose diet accumulated visceral fat and developed adiposity, which was paralleled with reduced hypothalamic Ob-Rb level and elevated SOCS3 and NPY levels. Interestingly, 10% fructose consumption led to enhanced glucocorticoid signalling and lipolysis in the adipose tissue, while consumption of 60% liquid fructose led to diminished glucocorticoid signalling, activation of adipogenic transcription factors and adipogenesis. In addition, fructose diet attenuated NFκB activation and did not alter proinflammatory cytokines in the adipose tissue, regardless of fructose concentration. In conclusion, these results propose that long-term fructose diet leads to hypothalamic leptin resistance and development of visceral adiposity, but only at higher fructose concentrations. The results also suggest that adiposity-related effects of fructose consumption may involve the interplay between leptin and glucocorticoid signalling at the level of central effector pathways that control energy balance.